Another option would be to use a Cycle Analyst or CA. The newer CA, version 3, is quite sophisticated and is a really useful tool even if all you do is hookup the shunt and only monitor your ebike. The CA really shines once you have it control the throttle. I've got my ebike to a point where I'm going to hook the CA to control my throttle. There is a 3 position switch that you can make a voltage divider out of and have the CA control the power levels or enforce max speeds. What I plan on doing is having the CA programmed with 3 different modes:
1. full throttle, just like you get from the factory
2. limit speed to 32km/hr
3. limit speed to walking speed
You can also change the throttle response and a lot of other things. The CA can really be quite daunting in its complexity to start with but IMO in the end it will be worth it. I know goldenmotor.ca carried the CA as that is where I got mine. Gary has a page on the hookup process here:
http://www.goldenmotor.ca/FAQ/questions.php?questionid=120PS. I hope this makes sense as I had very little sleep before writing this :-)